1. Field of the Invention
The present invention relates to a substrate for an inkjet head, an inkjet head, and an inkjet printing apparatus, and particularly relates to a substrate for an inkjet head, an inkjet head, and an inkjet printing apparatus wherein the insulation property of a protection layer is checked.
2. Description of the Related Art
An inkjet printing apparatus needs to continue to eject as large an amount of ink as possible with one heating resistor. However, heating resistors are occasionally subjected to physical action such as the impact of cavitation caused by ink foaming, shrinkage, and defoaming. Further, in a case where ink reaches the heating resistors, the heating resistors are occasionally subjected to chemical action of the ink.
There is a case where in order to protect the heating resistors from the above physical and chemical actions, a protection layer for protecting the heating resistors is provided. This protection layer is formed on or above the heating resistors, and needs to be made of a material having high heat resistance. Actually, the protection layer is formed with a metal film of Ta (tantalum), a platinum group element Ir (iridium) or Ru (ruthenium), or the like satisfying the above conditions.
The protection layer is normally in contact with ink. Accordingly, the protection layer is in a severe environment in which the temperature of the protection layer rises instantly because of the action of the heating resistors. In a case where electricity flows through the protection layer in this environment, an electrochemical reaction occurs between the protection layer and the ink and the entire protection layer is anodized or eluted. In a case where this phenomenon spreads, the protection film cannot play the original role and the other heating resistors are also ruptured soon.
There is a case where in order to prevent this, an insulating layer is provided between the heating resistors and the protection layer so that part of electricity supplied to the heating resistors does not flow through the protection layer. However, in a case where the electrically insulating protection film (insulating film) is defective at the time of manufacturing, there is a possibility that a short circuit will occur in a heating resistor layer, an electrode wiring layer, and the protection layer.
Accordingly, there is known a technique of connecting separate sections of an upper protection film via fuses each of which is blown in a case where the corresponding heating resistor is damaged (see, for example, Japanese Patent No. 3828728).
Incidentally, the upper protection film needs to play two roles. One role is to protect the heating resistors from the physical action and the chemical action, and this role is the original role of the upper protection film. In order to play this role, the upper protection film needs to have a certain level of thickness.
The other role is to form fuses as the upper protection film and in a case where one of the heating resistors is damaged, blow the corresponding fuse. The upper protection film is formed with high-melting-point metal and large energy is necessary to blow the fuse. Accordingly, it is desirable that the upper protection film be as thin as possible. In other words, the two roles have contradictory requirements for a film thickness.
In view of this, it is conceivable to provide individual through holes and form fuses in another wiring layer. However, since provision of the individual through holes requires space therefor, the density of arranged heating resistors becomes low and the area of a substrate for an inkjet print head becomes large.